Various types of cable have been used as distributed antennas. Two wire lines have been used in mines and tunnels as leaky feeder lines for communications, but have not found wide application because virtually all of their electromagnetic field propagates in the space around the cable and so is susceptible to environmental effects. Surface wave lines, such as Goubau lines, provide surface wave fields which are bound more tightly to the cable than those of a two-wire line but still are unduly susceptible to environmental effects. Leaky waveguides are less susceptible to environmental effects because their fields propagate almost entirely within the waveguide, but are generally limited to frequencies above several Gigahertz due to physical dimension constraints.
Distributed antennas in the form of leaky coaxial cables have been disclosed. Such leaky coaxial cables typically comprise a central conductor embedded in a dielectric material which is surrounded by one or more shields. Apertures in the shield(s) allow radio frequency energy to penetrate the shield(s) in a controlled manner. The size, shape and orientation of the apertures determine whether the cable supports a surface wave or a leaky wave mode of operation. Thus, in a paper entitled "Various Types of Open Waveguide for Future Train Control", Sumitomo Elect. Tech. Rev, June 1968 , Tsuneo Nakahara et al disclose a first open coaxial cable with numerous closely spaced apertures or a longitudinal slot for surface wave modes and a second open coaxial cable with zig-zag slots spaced at intervals of about one wavelength for conveying leaky wave modes.
Leaky coaxial cables used in intrusion detection systems preferably set up an electromagnetic field around the cable which decays rapidly with radial distance. This rapid decay rate is very desirable for a cable to be used as a distributing antenna in an intrusion detection system because it provides a well-defined detection zone. An intruder moving in proximity to the cable, say within one meter, disturbs the electromagnetic field coupling the cable and is thereby detected, whereas a person or vehicle moving sway, will not be detected.
The electromagnetic field produced by such a cable is the sum of the field produced by each aperture taking into account all external modes of propagation, including radiation away from the cable and transmission line along the cable. The attenuation and velocity of propagation of the transmission line modes are highly dependent upon the medium surrounding the cable. A disadvantage of leaky coaxial cable is its susceptibility to mode cancellation effects when the cable is used in a low loss environment such as when it is mounted in air. If the cable is mounted in air, transmission line modes propagate at almost the velocity of free space and with minimal attenuation. On the other hand, if the cable is buried in soil, the transmission line mode propagates relatively slowly and with considerable attenuation. Hence, in a low loss soil such as dry sand, perhaps 95% of the field at a given distance from the cable will be due to approximately 10 meters of cable, while in heavy clay it might be due to less than 1 meters of cable. Hence, the field at a given distance from the cable will decrease rapidly as soil loss is increased. Consequently, buried leaky coaxial cable sensors are very susceptible to soil conditions and environmental conditions.
If a leaky coaxial cable is laid on the soil surface or mounted parallel to the soil surface or another conductor, for example a wire fence, mode cancellation effects may be experienced due to "image line" fields being set by in the soil or other conductor. Also, discontinuities in the field of the image line can cause reflections which cause radiation and standing waves, further corrupting the transmission line mode. The end result is a very erratic external field which is strongly influenced by its surroundings and physical motion of the cable relative to the soil or other conductor. As a result of these problems, the use of leaky coaxial cable sensors has been generally limited to buried applications where the attenuation of the externally propagating surface wave is sufficient to prevent mode cancellations.
In order to overcome these disadvantages, it has been proposed in European patent application No. EP 0,322,128, and United States equivalent U.S. Pat. No. 4,987,394, to provide a special helical winding of fine steel wires on the outside of the outer shield of the leaky coaxial cable. In effect, the resistive properties of the helical outer conductor have the same effect as if the cable were buried in a lossy medium. While this cable offers advantages over other leaky coaxial cables, it is inherently expensive to manufacture and not entirely satisfactory because the magnetic field lines of the helical winding are not compatible with an axially cylindrical surface wave. The cable described in EP 0,322,128 would support a multiplicity of radiating and propagating modes and hence not be entirely suitable for use in air. In one embodiment, EP 0,322,128 also discloses forming the shield as a plurality of discrete sleeve elements overlapping each other in fishscale fashion. The predominantly capacitive coupling between these overlapping sleeve elements would tend to speed up transmission line mode propagation, which would exacerbate the problem by causing leaky wave radiation.